1. Field of the Invention
The present invention relates to smoking articles and, more particularly, to smoking articles having filter elements which contain means to reduce vapour phase constituents of smoke from the smoking articles.
2. Brief Description of Related Art
The use of carbon or activated carbon in tobacco smoke filter elements to reduce vapour phase constituents of smoke has been known for some while. Commonly, carbon has been utilised either in a dual filter arrangement, the carbon granules being sprinkled onto sticky cellulose acetate tow, which tow is gathered in conventional manner and cut into double unit lengths. The double unit lengths of carbon containing acetate are then interdigitated with plain cellulose acetate filter elements having double unit lengths. The interdigitated assemblies are wrapped in plugwrap and then cut in the mid-point of both the carbon-containing filter element double unit length and the plain cellulose acetate double unit length to provide wrapped filter elements having a carbon-containing section adjacent a non carbon-containing section. This type of filter is known as an active acetate or "AA" filter.
In the alternative, carbon has been utilised in a triple filter arrangement either with the carbon being incorporated in the cellulose acetate tow, as described above, and in UK Patent Specification No. 1,087,909, or with the carbon being freely held in a cavity between two plugs of tobacco smoke filtration material, such as cellulose acetate, and described in U.S. Pat. No. 4,185,645. Another alternative and commercially produced carbon filter is the ACT (Active Carbon Thread) Filter made by Filtrona UK, where the carbon in the centre section is adhered to a cotton thread and then surrounded by cellulose acetate. The carbon thread section offers the path of least resistance and the majority of the smoke passes through the carbon centre.
A disadvantage with these arrangements is that most, if not all, of the tobacco smoke is passed through that section of the filter element containing the carbon.
UK Patent Specification No. 1,410,048 sought to overcome the problems associated with carbon `off-taste`, a feature commonly found with carbon-containing filter cigarettes where all the smoke passes through the carbon section. There was therein discussed and filters were produced which comprised longitudinally extending regions separated by diaphragm(s) of vapour-porous material, one region comprising a flow path for smoke and one region containing carbon and being closed to smoke. FIG. 1 of that document is reproduced in the drawings hereof under the designation FIG. 1 (Prior Art). This embodiment shows a triple filter element having a filter section 1 comprised of two concentric tubes 5 and 6, inner tube 6 comprising highly vapour-porous paper (&gt;1,000 Coresta Units) and outer tube 5 comprising a smoke-impervious wrapper. The annular space 7' is filled with carbon particles and the interior of the inner tube 6 is empty and forms a central smoke channel 7. The annular space 7' is blocked at both ends with impervious annular closures 8. At either end of filter section 1 are filter plugs 2 and 3 of cellulose acetate. In use, smoke is drawn through the smoke channel 7 without flowing through the carbon material. Volatile constituents are reduced owing to the diffusion of the volatile constituents of the smoke through the inner tube 6 into the carbon-filled annular space 7' in which they are absorbed. There is no carbon off-taste as the particulate matter of the smoke does not come into contact with the carbon material.
The filter section 1 of UK Patent Specification No. 1,410,048 was produced by sprinkling carbon granules over the whole of the inner surface of a plugwrap and wrapping the core element of the inner tube 6 with the carbon-treated plugwrap. These cut filter sections were then abutted, together with the annular disc blocking closure 8, to the end of filter plugs 2 and 3 in conventional triple filter manner.
A disadvantage with this arrangement is that it is difficult to produce such a filter which has the ends of the annular space blocked in a satisfactory manner and to assemble the multiple components at high production speeds.
Furthermore the filtration efficiency of conventional carbon-containing triple filters for vapour phase constituents is only of the order of about 20% for acetaldehyde and acrolein, for example.